With the advance of science and technology, people rely on electrical energy more and more, and electrical power has been a dispensable resource for modern life. Before, people were to be contented as long as there was no scarcity of electrical power. However, owing to the uprise of the living standard and the upgrade of the scientific-technological industry, high-quality power supply has been the common target of all countries. In many countries, the traditional industry has been evolved into the high-technology and high-added-value industry, and it means that various precision equipments have been extensively used; therefore, the requirement of electrical power has also changed, and in addition to purchasing the uprise of power supply quantity, users also pay much attention to the quality of power supply. For power supply quantity, building a multitude of power plants is not the only way to solve the power problem; promoting the power factor or the power efficiency of various electrical products is also an effective method. At present, most electrical equipments utilize direct current directly or indirectly; however, owing to generator systems and the need of power transmission, power plants provide alternating current. Therefore, users have to transform alternating current into direct current with AD/DC converter. Owing to low cost and simple structure, the common AD/DC converter is the diode bridge rectifier, which needs only four diodes. Refer to FIG. 2 for the voltage/current waveforms of the diode bridge rectifier. This kinds of circuit has the disadvantages of the great harmonic component of the input current, which causes the serious distortion of the input current waveform, and the phase difference between input voltage and input current, which causes the reduction of power factor, and those disadvantages will further causes power system instability or even power supply interruption. Owing to the characteristics of the internal impedance, the power factors of many current electrical devices are pretty low; however, users demands power supply quality more and more strictly now; therefore, the improvement of the power factor of power supplies become an important subject, and the technology thereof focuses on the power filter circuits of power supplies.
The main function of a power filter circuit is to make voltage and current in-phase and make a load perform like a resistor in order to reduce the harmonic component of input current, and the abovementioned function can be implemented with various circuit designs, which can be divided into passive power filter circuits and active power filter circuits. The industries usually adopt the passive power filter circuit because of its simple circuit and low cost. The passive power filter circuit is primarily composed of passive elements, such as capacitors and inductors, which are used to offset the advance or the lag of the current phase and to reduce the harmonic component of current. When the requirement of power factor is not so strict, the inductor formed of a plurality of gap-spaced silicon steel sheets is coupled to the input terminal in series, or an inductor cooperates with a capacitor to form an LC-type or π-type low pass filter. However, the lower the frequency used, the greater the inductance needed; for example, if the ATX power supply of a personal computer is a passive filter circuit, it is often big and heavy, and the best power factor thereof is only as high as 70%. Thus, for a strict power factor demand, the passive power filter is not suitable.
In order to improve the inferior power factor of the passive power filter circuit, the conventional technologies adopt a compensating circuit, installed to the original passive power filter circuit. Referring to FIG. 1, the conventional passive power filter circuit comprises: an overload protection circuit 11, a surge current-limiting circuit 12, a first filter circuit 13, a power factor regulating circuit 14, a rectifier 15, a power factor compensating circuit 16, a second filter circuit 17, a power source push circuit 18, a main voltage transformer 19, an output rectifier 20, a power source feedback circuit 21, a standby power supply circuit 22, and output filter circuits 23, 24, 25. The power factor compensating circuit 16 is composed of two diodes D4, D5 connected in series and a capacitor C8. The power factor compensating circuit 16 needs a voltage supply to provide a rated voltage for the capacitor C8 in order to pre-charge filter capacitors C5, C6 and obtain a compensating effect; therefore, in addition to being a standby power source and providing power for a control IC 211, the standby power supply circuit 22 bypasses a loop from the cathode of the diode D6 thereof to provide power for the power factor compensating circuit 16 so that the power factor compensating circuit 16 can charge the capacitor C8, and thereby, the objective of compensating power factor is achieved, and the output power factor is improved. However, the power factor compensating circuit 16 can only obtain an unchangeable rated voltage via the supply voltage bypassed from the standby power supply circuit 22 to the power factor compensating circuit 16; further, the control IC 211 of the power source feedback circuit 21, which usually needs a specified power that cannot be altered arbitrarily, will be interfered owing to the bypass of the standby power supply circuit 22; thus, the power source feedback circuit 21 may malfunction, and the power supply output may become instable.
In conclusion, the passive power filter circuit has the disadvantages of noise, working frequency vibration, and low power conversion efficiency, and the passive power filter circuit installed with the conventional compensating circuit still has the problem of instable power output. Therefore, there is still room to improve the passive power filter circuit in spite of its extensive application.